1. Field of Invention
The present invention relates to a charge control circuit, which is capable of supplying power to a system while simultaneously charging a battery. The voltage at a supply terminal is feedback controlled according to the voltage and current sensed at a charging terminal, and the conduction status of a transistor between the supply terminal and the charging terminal is controlled according to the voltage sensed at the supply terminal.
2. Description of Related Art
Rechargeable battery is widely used in various portable electronic devices such as: tablet computer, smart phone, etc. Because the battery is charged while power is being supplied to the system, it is required for the relation among an external power source, a system input voltage, and the battery voltage to meet certain requirement. That is, when the external power source supplies power to the system and charges the battery, current should not flow back from the battery to the system, otherwise the battery will lose electricity and can not be fully charged. Usually, it is designed such that there is a minimum voltage difference between the battery voltage and the system input voltage when the battery is being charged, and this voltage difference is predetermined during circuit design.
Refer to FIG. 1, which shows the relation between the voltage at the charging terminal Vbat (to battery) and the voltage at the supply terminal Vsys (to system), when an external power source Vin supplies power to them. A voltage difference Vos is predetermined to keep a margin between the charging terminal Vbat and supply terminal Vsys. FIG. 2 shows a simplified schematic diagram according to prior art U.S. Pat. No. 7,710,079 which implements the relation shown in FIG. 1. As shown in FIG. 2, a charge control circuit 100 includes a regulator 101, a charge control unit 102, an error amplifier 103, an internal voltage source 104, a transistor Q100, a supply terminal Vsys, and a charging terminal Vbat. The regulator 101 is coupled to an external power source Vin and supplies power to a common node N101. The transistor Q100 is coupled between the common node N101 and a common node N102. The charge control unit 102 is coupled to the gate G100 of the transistor Q100. The internal voltage source 104 is coupled between the common node N102 and the error amplifier 103 to provide a voltage difference Vos in between. The error amplifier 103 includes a negative input terminal and two positive input terminals, wherein the negative terminal is coupled to the common node N101, and two positive terminals are respectively coupled to the internal voltage source 104 and a reference voltage Vref; the reference voltage Vref can be set in correspondence to the voltage 3.4V shown in FIG. 1. When the charge control circuit 100 is in the charge phase I of FIG. 1, the sum of the voltage at the charging terminal
Vbat and the voltage difference Vos is less than the reference voltage Vref ((Vbat+Vos)<Vref), so the error amplifier 103 compares the voltage at the supply terminal Vsys and the reference voltage Vref to generate a control signal, which regulates the power supplied from the regulator 101 at 3.4V. When the charge control circuit 100 is in the charge phase II, that is, the sum of the voltage at charging terminal Vbat and the voltage difference Vos is larger than the reference voltage Vref ((Vbat+Vos)>Vref), the error amplifier 103 compares the voltage at the supply terminal Vsys and the voltage at the other positive input terminal (Vbat+Vos, the sum of the voltage at the charging terminal Vbat and the voltage difference Vos) to generate a control signal, which regulates the power supplied from the regulator 101 to maintain a predetermined voltage difference Vos between the supply terminal Vsys and the charging terminal Vbat.
The foregoing circuit design has a drawback that the voltage difference provided by the internal voltage source is a fixed predetermined value. In practical circuit operation, it suffices as long as the voltage difference can keep the voltage at the charging terminal to be lower than the voltage at the the supply terminal so that no current will flow from the battery back to the system. However, the predetermined value of the voltage difference is often larger than required, because the parameters are often set by a conservative point of view in circuit design. In other words, the fixed predetermined value is not the optimal value and may cause unnecessary power consumption.